Conventionally, a medical stent (hereinafter also abbreviated as ‘stent’) is placed to a stricture in a lumen inside a living body such as a blood vessel, a digestive tract, a bile duct, a pancreatic duct, or a urinary duct, for example, in order to expand this stricture and/or maintain an open state. Stents are generally thin walled tubular-shaped devices composed of complex patterns of interconnecting struts which function to hold open a segment of such lumens. Commercially available stents are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. A stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed into a radially-expanded configuration. In the case of a self-expanding stent, deployment is achieved by the removal of a constraint, such as the retraction of a delivery sheath. In the case of a balloon expandable stent, deployment is achieved by inflation of a dilation balloon about which the stent is carried on a stent-delivery catheter.
A stent should have adequate radial strength (i.e., hoop strength) to withstand structural loads, particularly radial compressive forces imposed on the stent as it supports the walls of a vessel lumen, even in the event of vessel spasm, and it should be longitudinally flexible to allow it to be maneuvered through a tortuous path and to enable it to conform to a site that may not be strictly linear or may be subject to flexure. The material from which a stent is constructed should allow the stent to undergo radial expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, a stent should maintain its size and shape throughout its service life. Also, the material of which the stent is made should be biocompatible so as not to trigger any adverse vascular responses.
In addition to these requirements, a stent should also be viewable under radioscopy. This is because the position of the stent needs to be confirmed by fluoroscopy or fixed X-ray equipment while placing the stent into position, after it is positioned, as well as during removal, if necessary. Thus, a stent should also be radiopaque to allow for real time visualization. That is, a stent or components thereof should block or attenuate the passage of X-rays more than the surrounding tissue.
This is typically accomplished by the use of radiopaque materials in the construction of a stent, which allows for its direct visualization. The most common materials used to fabricate stents are stainless steel and nickel-titanium alloys, neither of which is particularly radiopaque. This factor, in combination with the relatively thin wall thickness (about 0.002 to 0.006 inch) of most stent designs renders stents produced from these materials insufficiently radiopaque to be adequately visualized with fluoroscopy procedures. Alternatively, high density biocompatible metals, such as tantalum, iridium, platinum, gold, and the like, may lack suitable physical properties, e.g. flexibility, elasticity, tensile strength, may be too costly, and may be excessively radiopaque. Also, stents constructed of only highly radiopaque materials may appear overly bright when viewed under a fluoroscope, thereby obscuring visualization of the stented lumen. Thus, stents have been designed that combine different materials to produce a mechanically sound, biocompatible, and fluoroscopically visible stent.
One approach that has been used to increase the radiopacity of stents is through attaching radiopaque markers to the stent. Using such markers of sufficient size and quantity distributed over the body of the stent can provide a pixelated or compound image that informs the clinician of the location, orientation, and shape, e.g. degree of deployment of the stent in the patient. Radiopaque markers, however, may project from the surface of the stent. If the markers project inwardly, fluid flow may be disrupted. If they project outwardly, the wall of the lumen wall tissue may be damaged. In an alternative approach, the radiopacity of stents has also been increased by plating or coating selected portions thereof with radiopaque material. However, under certain conditions cracks may form in the plating or coating causing portions of the plating to separate from the underlying substrate. This has the potential for creating jagged edges that may inflict physical trauma on the lumen wall tissue or cause turbulence in the blood flowing past the stent, thereby inducing thrombogenesis. Thus, composite stents, whether equipped with markers or radiopaque plating, have several disadvantages; namely, separation of the markers, plating, or coating from the substrate material, which may allow the metallic particles to flow downstream within a vessel lumen causing potential blockages or other adverse effects upon the patient. One approach to overcome these shortcomings of composite devices is a stent formed from a single material that possesses the required mechanical and radiopaque properties. An example of such a material is a binary alloy of either tantalum-tungsten or tantalum-niobium; however, the cost and complexity of manufacture of such devices is undesirable.